Mongo is great for when you're rapidly developing a product and/or still operating at a small-medium scale.
Once you start to really scale, you'll start to find a couple parts of MongoDB break-down:
1. MongoDB has no concept of transactions and is not ACID compliant. These short-comings seem innocuous enough at first, but you end up with some crazy potential race conditions that you just end up praying you never face.
2. Distributed MongoDB layers are difficult to implement as well as maintain.
3. MongoDB Replica sets are unpredictable and unreliable in the speed at which they are able to stay up to date and require changes to your code to fully utilize (since you need to ensure you are always reading from a replica when you can, but only ever writing to the master MongoDB instance)
4. At scale, MongoDB will consistently perform worse than Postgres for CRUD-based operations
5. Do you have true relations between documents? Do you ever use Mongoose's convenient `populate()` functionality? If so, you are now making multiple db queries in series when trying to fetch a document(s) from a single collection. This starts to really hurt your query times once you get enough documents/relations in your mongo collections.
I'm sure I'm missing some, but these are some of the areas where MongoDB has fallen down for me in the past.
